Lamellar boundary slip in polyethylene

Mechanical relaxation in polyethylene specimens prepared in various ways has been studied at 0.67 c/s and by creep experiments of 35 mm duration in order to elucidate the mechanisms of the $\alpha$ and $\alpha'$ relaxations. Internal friction measurements were made from 20 to 120$^\circ$C and creep measurements from 14 to 85$^\circ$C. The $\alpha$' relaxation has five significant properties. (1) The magnitude of the $\alpha$' relaxation is highly sensitive to anisotropy in the orientation of lamellae. (2) The $\alpha$' retardation time can be increased by irradiation which cross links the molecules in between lamellae. (3) The $\alpha'$ retardation time is greater in a slowly crystallized specimen with coarse lamellae than in a quickly crystallized specimen with fine lamellae. (4) The $\alpha'$ creep is entirely recoverable. (5) The superposed creep curves form not one master curve but two. These properties are predicted by a model in which the $\alpha'$ relaxation is due to slip at lamellar boundaries while the $\alpha$ relaxation occurs within the lamellae. The model predicts full recoverability of the relaxation and the retardation time of the $\alpha'$ relaxation to be a function of the compliance of the lamellae, the viscosity of the inter-lamellar material and the geometry of the lamellae. The experiments also proved that large variations in the spherulitic structure of polyethylene have no measurable effect on any of the properties of the $\alpha$ relaxation. The $\alpha$ relaxation cannot therefore be due to slip at the spherulite boundary.